This invention relates to safety features for switches, and particularly for ground enclosed, load-break switches adapted for use in medium voltage power distribution systems in the range about 1 to about 36 kilovolts (kv) for interrupting currents of up to about 1 kiloampere (ka).
Load break switches used in medium voltage power distribution range circuits generally include a pair of electrodes, one being stationary and the other movable to open and close the circuit. As commonly used in three-phase systems, three or multiples of three switches are mounted in a common grounded metal enclosure.
One type of load break switch for power distribution systems is gas insulated switches that employ a gas for both insulation and interruption. Sulfur hexafluoride (SF.sub.6), either alone or mixed with other gases such as nitrogen, is used. The gas is used to rapidly extinguish the arc formed as the switch is opened. In a typical three phase configuration, a grounded metal enclosure surrounds three, or multiples of three, switches. Each individual switch typically comprises an unsealed cylindrical housing of a plastic such as reinforced epoxy resin. A grounded metal housing filled with sulfur hexafluoride surrounds the interrupters with substantial clearance to prevent arcing.
A problem with pressurized gas filled switches is that the pressure within the enclosure can degrade to an unsafe level at which the arc developed upon opening the switch might not be quenched, resulting in rapid heating and vaporization of the contacts, and in some instances, an explosion. A pressure gage can be provided, but this does not prevent opening the switch and it constitutes an additional source of leakage.
An additional problem with sealed switches is that a malfunction within the switch can cause uncontrolled arcing, heating and consequent vaporization of metalic contacts. This increases the internal pressure of the switch and creates a safety hazard of a possible explosion of the switch.
In view of these problems, there is a need for a pressurized switch that is not a safety hazard when the gas pressure within the switch is either too high or too low and does not incorporate unnecessary sources of leakage.